3-Fluoro-, 2-and 3-chloro-quinolines were deficient in mutagenicity in S. typhimurium TA100, whereas all the other fluoro and chloro derivatives examined were mutagenic. Together with mutagenic property of methylquinolines, the mutagenic metabolite of quinoline is presumed to be the 2, 3-dihydro-2, 3-epoxide of 1, 4-hydrate form of quinoline. A possibility is proposed that a fluorine substitution may deprive the aromatic compounds of the genotoxicity.